The overall objective of the proposed research is to gain a better understanding of the in vivo control of the biosynthesis of deoxynucleotide triphosphates, the precursors of DNA. A key control point is in ribonucleotide reduction. An attempt will be made to determine the relationship between the synthesis of the four protein components of this system and the concentrations of various possible corepressors (dTTP,dATP, dGTP. dCTP and cysteine) by the use of various mutant strains. Also included in the proposed research is the isolation and characterization of mutants defective in the operator and regulator genes controlling the ribonucleotide reductase system as well a mutants which produce a ribonucleoside diphosphate reductase with altered allosteric regulation. Mutants which produce a defective thioredoxin or a defective thioredoxin reductase, will be isolated to determine the role of these proteins in metabolism other than in ribonucleotide reduction. A further objective is to isolate and characterize mutants defective in dUTP pyrophosphorylase and deoxynucleotide kinases. The former class would be useful to determine the mechanisms that are used to exclude this compound from incorporation into DNA. The latter class would be useful to determine specificities of these enzymes.